Don't generate exit events for jobs created from within event handlers

Add a new job property from_event_handler, and do not create exit events for
such jobs. This prevents easy accidental infinite recursion.

src/fish.cpp

@@ -520,8 +520,6 @@ int main(int argc, char **argv) {
 
     int exit_status = res ? STATUS_CMD_UNKNOWN : parser.get_last_status();
 
-    // TODO: The generic process-exit event is useless and unused.
-    // Remove this in future.
     event_fire(parser,
                proc_create_event(L"PROCESS_EXIT", event_type_t::exit, getpid(), exit_status));